Alkynes vinylcuprate intermediates

As a result of this process, the sequential addition of a carbon nucleophile and a carbon electrophile across a triple bond is achieved. For example, addition of cuprate 101 (Scheme 2.32) across the triple bond of acetylene produces the vinylcuprate intermediate 102. Quenching of the latter with electrophile 103 gave acetoxydiene 104, the active constituent of the pheromone Cossus cossus. The sequence exemplified in Scheme 2.32 enables independent variations in the structure of all participants involved, namely the alkyne, the organome-tallic nucleophile, and the electrophile. Therefore this approach can serve as a unified protocol for the one-pot assemblage of various alkenes from simple precursors. 

This mixed cuprate is useful for preparation of 2-methyl-l-alkenes from 1-alkynes (equation II). In the absence of LiBr the intermediate vinylcuprate a decomposes to... 

Various functionalized alkynes can be submitted to carbocupration reactions, such as alkoxyalkynes,150 alkynyl carbamates,151 acetylenic orthoesters,152 and thioalkynes.153 The carbocupration of orthoesters, for example, 204, has been used to prepare a-substituted esters of the type 206 by acidic hydrolysis of the adduct 205 (Scheme 51).152 This allows the formation of regioisomers that are not accessible by copper-mediated addition to acetylenic esters. A stereoselective synthesis of trisubstituted alkenes has been described by Normant et al.lSd> starting from phenylthio-acetylene 207. Carbocupration with lithium di- -butylcuprate affords the intermediate 208 which, upon addition of /z-butyllithium, undergoes a 1,2-metalate rearrangement to the vinylcuprate 209. The latter can be trapped with various electrophiles, for example, ethyl propiolate, providing product 210 with complete regio- and stereocontrol. 

---